Alesis QuadraSynth Plus Reference Manual page 150

Reverb (Page 4)
QuadraSynth Plus Piano Reference Manual
Density
Density controls how the first reflection of the reverb effect will
appear. When set to 0, the first reflection is heard alone without
any other reflections. When set to 99, the first reflection appears
to "fade-in" and then "fade-out". This is because a number of
reflections will occur just before and just after the first reflection,
in addition to the remaining reflections heard after the first
reflection. Thus, the reverb sounds more "dense".
If the select Reverb type is Large, the Density parameter is
unavailable.
Diffusion
Diffusion determines the "thickness" of the reverb sound by
adding more reflections to the reverb's decay. With lower
diffusion settings, you may be able to actually hear the
individual echoes that make up the overall reverb sound. With
higher diffusion settings, the echoes increase in number and
blend together, washing out the reverb's decay. Greater
diffusion works better with percussive sounds, whereas less
amounts of diffusion work well with vocals and other sustained
sounds.
Note: The illustration above reflects a Density setting of 0.
Editing Effects: Chapter 7
151